The present invention relates to an automatic gain control (AGC) circuit used in the signal receiving section of a video data transmitting device such as a facsimile system. More particularly, the invention relates to a keyed AGC circuit which can carry out sensitive operations stably.
Keyed AGC systems have been employed for AGC circuits in facsimile systems because these circuits are very little affected by noise components in an input signal. An example of a conventional keyed AGC system is shown in FIG. 1. An electronic switch 1 is closed only for the synchronization period for each scanning line thus forming a closed loop only during such periods. Upon formation of the closed loop, a part of an output signal 9 is applied through a full-wave rectifier circuit 4 to one input terminal 12 of a level comparator 5 where it is compared with the level at the other input terminal 13 which is preset such as with a potentiometer 7. According to the comparison result at the output of the level comparator 5, a potential holding capacitor 6 is charged or discharged. The potential of the potential holding capacitor 6 is applied through a buffer 2 to a multiplier 3 to thereby set an amplification factor of the multiplier 3 so that, even when the level of an input video signal 8 to the multiplier 3 varies, the level of the output signal 9 is maintained unchanged. In this operation, the period that the closed loop is formed is about 5% of each line scanning period T as shown in FIG. 2 and the amplification factor which is set by the potential of the potential holding capacitor 6 is maintained for a video signal transmission period corresponding to the remaining 95%.
With this circuit construction, it is difficult for the conventional keyed AGC circuit to operate stably because of the variations in potential of the potential holding capacitor 6 due to external noise. Furthermore, in data transmission in a facsimile system, sometimes the amplification factor is not set to the correct value when the closed loop is formed because of momentary interruption of signal transmission circuits.
In addition, with the conventional keyed AGC circuit, it is technically difficult, where the signal level during the synchronization period of the input video signal 8 is greatly varied, for the gain of the multiplier 3 to be controlled precisely thereby and therefore difficult to perform stable operations.
Accordingly, an object of the invention is to provide a keyed AGC circuit in which the above-described difficulties accompanying a conventional circuit have been eliminated and in which stable operations are effected.